1. Field of the Invention
This invention relates to displacement measurement devices and more particularly to devices wherein light is utilized to accurately sense very small relative displacement between two objects.
2. Description of Prior Art
There are numerous occasions when it is desireable to sense very precisely any relative displacement between two objects. Often it is desireable that the accuracy of measurement of such displacement be independent of ambient conditions such as temperature, moisture, etc. Many attempts have been made to provide such precise measurement with varying degrees of sucess. The existing art consists of three basic types of devices: those employing interferometer techniques, those employing optical gratings, and those using optical resonant cavities.
An example of an interferometer type of device is disclosed in U.S. Pat. No. 3,666,371 issued to Hendrik de Lang on May 30, 1972. Such devices typically require at least two optical arms requiring accurate positioning. Also the interferometer approach typically employs multiple optical surfaces between the light source and the measurement surface giving rise to the potential of optical distortion from the optical surfaces.
Examples of grating devices include U.S. Pat. No. 3,364,813 issued to R. L. McKinney Jan. 23, 1968; U.S. Pat. No. 4,115,008 issued to Alexander Turnbull Shepard Sept. 19, 1978; U.S. Pat. No. 4,079,252 issued to David George Brake Mar. 14, 1978 and U.S. Pat. No. 4,266,535 issued to Knut Heitmann et al Oct. 7, 1980. Typically these devices are accuracy limited because the measurement accuracy is determined by the grating which is susceptible to environmental effects and external damage. Furthermore, it is difficult to maintain grating accuracy while mounted on a moving object. The maximum distance measured with the grating type of device is limited by the grating length itself.
Examples of resonant cavity devices include U.S. Pat. No. 3,476,483 issued to Richard F. Weeks Nov. 4, 1969 and U.S. Pat. No. 3,514,207 issued to Hendrik de Lang et al May 26, 1970. These devices require a two port laser. They also typically require a cavity for each measurement device and a beam splitter to detect direction of motion.
All three approaches typically utilize a simple amplitude photodetector for measuring the light beam. This type of detector essentially absorbs the incident light beam completely.